54 THE EVOLUTION OF CONTINUITY 



In this figure, A and &, the division-results of X, 

 a zygote, respectively originate lines of cells of male and 

 female gamete potentialities. A and & are attached end to 

 end and compose a filament of two cells. Let us follow the 

 development of A. It grows to divide into B, B. But 

 both these two do not straightway grow to divide ; the 

 proximal B, that is the one next to &, is arrested, and it is 

 distal B which presently divides. The filament is now 

 composed of &, B, C, C (for the sake of convenience we are 

 not allowing & to do anything). Proximal C is now arrested, 

 while distal C becomes D, D. In the same way distal D 

 becomes E, E, and then distal E becomes F, F. Finally 

 distal F becomes G, which we will take to represent a gamete. 

 Thus the complete series resulting from A is B, C, D, E, F, G. 



As a rule gametes are produced singly at intervals in the 

 length of the Spirogyra filament, and not in pairs as one 

 might expect seeing that the division-results of distal F are 

 identical penultimate cycle-stages ; and the presumption is 

 that distal F is alone allowed to carry on the further develop- 

 ment necessary to give it its true gamete characters, proximal 

 F remaining arrested in a pre-gamete condition. 



The further development of the filament in the figure 

 will depend in its details on the plan followed with respect 

 to release from arrest. We shall suppose that this release 

 takes place on a regular plan, and that one after another 

 the successive cells of a series are set free to produce new 

 series. Thus if we let B (Fig. 6, 6) grow to divide we would 

 have the result drawn in the next figure. 



According to our hypothesis, B will become a new series 

 C, D, E, F, G (Fig. 7, 7) ; and this series will be intercalated 

 between & (which was not allowed by us to develop) and C 

 of the first series. In Spirogyra all newly formed cells are 

 intercalated in the single series, and there is no lateral 

 branching. 



The intercalated series is shorter by one cell than 

 B, C, D, E, F, G, and as it takes the place of B the result 

 is that there is now a series of C, D, E, F, G, C, D, E, F, G. 

 Similarly, the release from arrest of the two C's would bring 

 about the intercalation of a new series in place of each ; that 

 is, D, E, F, G, D, E, F, G, as in Fig. 7, 8. 



By developing on such a system Spirogyra would form in 



